An Infiltration Exercise for Introductory Soil Science
K. A. Barbarick,* J. A. Ippolito, G. Butters, and G. M. Sorge
ABSTRACT
One of the largest challenges in teaching introductory soil
science is explaining the dynamics of soil infiltration. To aid students in understanding the concept and to further engage them
in active learning in the soils laboratory course, we developed an
exercise using Decagon Mini-Disk Infiltrometers with a tension
head (ho) of 2 cm. Groups of up to four students measured cumulative infiltration for up to 5 minutes and plotted their results
vs. time (t). Curves for each group were plotted using SigmaPlot
through a computer-projection system so that students could
observe infiltration differences between different soils. With the
help of their lab instructors, regression analyses were completed
to obtain curve-fitting parameters needed to calculate hydraulic
conductivity (K). Upon completion of data analyses, the students
worked cooperatively to answer several questions concerning
infiltration physics. Based on student comments, we believe that
we successfully engaged the students and increased their exposure and understanding of infiltration dynamics.
B
ecause of the quantitative nature of teaching soil–water dynamics in introductory soil science courses, tools that allow students
to observe physical phenomenon usually aid the instructor in teaching these concepts. Thien (1973) placed enough importance on infiltration and runoff that he developed it as 1 of 30 mini-courses used
to supplement introductory soil science lectures. Helsel and Hughes
(1984) found that 1 of 36 areas where urban students felt less competent than rural students was the ability to “explain the importance
of soil physical properties on infiltration, percolation and runoff.”
Their solution to building the urban student’s confidence was the
production of 7- to 8-minute videotapes on each topic.
The specific physics and math required to quantify infiltration or
hydraulic conductivity are beyond the capability of most introductory soil science students. The use of Decagon Mini-Disk Infiltrometers (cost ~$50 each; Pullman, WA), however, allows students to
observe and graph infiltration dynamics for different soil textures,
statistically determine empirical parameters, and calculate hydraulic
conductivity.
Decagon Devices (1998) recommends utilizing the approach of
Zhang (1997) to determine hydraulic conductivity from the cumulative infiltration measured with one of their infiltrometers. Zhang
(1997) used a truncated Philip-type expression (Philip, 1957) to
quantify cumulative infiltration as follows:
I = C1t 0.5 + C2t [1]
where I is the cumulative infiltration (cm), t is time (seconds), C1 and
Department of Soil and Crop Sciences, 200 W. Lake Street, Colorado State
Univ., Fort Collins, CO 80523-1170. Received 25 Mar. 2005. *Corresponding author ([email protected]).
Published in J. Nat. Resour. Life Sci. Educ. 34:72-76 (2005).
http://www.JNRLSE.org
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
72 • J. Nat. Resour. Life Sci. Educ., Vol. 34, 2005
C2 are adjustable parameters related to sorptivity (S) and hydraulic
conductivity (K), respectively, at the pressure head ho maintained by
the infiltrometer. Considering here only K(ho), Zhang (1997) showed
that the conductivity could be simply estimated as follows:
K(ho) = C2/A [2]
where A is a soil texture dependent dimensionless coefficient determined empirically. Values of A have been tabulated for a wide range
of soil textures (Table 1) assuming the van Genuchten type moisture
retention function (van Genuchten, 1980) with texture class parameters according to Carsel and Parrish (1988). The values in Table 1
correct those provided by Decagon that used the tension head (e.g.,
2 cm) instead of the pressure head (e.g., −2 cm) as required in Eq.
[21] and [22] of Zhang (1997). A pressure head of −2 cm is close
enough to saturation (ho = 0 cm) that we would still have rapid flow
while preventing the need for a retaining ring to contain surface
ponding at ho = 0 cm.
An illustration of the anticipated infiltration characteristics is
helpful in introducing the laboratory exercise. Figure 1 shows a
two-dimensional representation of the soil wetting after 5 minutes
of infiltration (Fig. 1A) and the corresponding experimental results
the students can expect (Fig. 1B). Noting that at any time the slope
of the I vs. t graph is the infiltration rate, students are instructed to
expect higher rates in the coarser-textured soil. By the combination
of matric and gravitational forces, the water movement is outward
and downward from the infiltrometer with the greatest depth of wetting in the coarsest (highest K) soil (Fig. 1A). Students should be
encouraged to verify this with a visual inspection of the wetted soil
volume at the conclusion of the data collection.
MATERIALS AND METHoDS
Properties of the four soils used in our exercise are listed in Table 2. We prepared air-dried soils by crushing them to pass a 2-mm
sieve. We then added each soil to separate 5.3-L (30 cm long by 19
cm wide by 9.3 cm tall) Rubbermaid Plastic Clear tubs to approximately 75% of the tubs’ capacity. The students then completed the
exercise provided in Table 3.
We used a small, slotted scoop to remove the wetted Terry very
fine sandy loam soil (coarse-loamy, mixed, superactive, mesic Ustic
Table 1. Texture dependent dimensionless coefficient in Eq. [2] according to Zhang (1997).
Soil texture
Sand
Loamy sand
Sandy loam
Loam
Silt
Silt loam
Sandy clay loam
Clay loam
Silty clay loam
Sandy clay
Silty clay
Clay
A parameter (Eq. [2])
(ho = −2 cm)
infiltrometer radius = 1.59 cm
2.37
3.33
5.36
8.60
11.95
10.88
5.82
9.11
11.67
5.61
8.72
5.90
Fig. 1. (A) Two-dimensional numerical simulation of infiltration wetting front in contrasting soil textures. (B) Corresponding cumulative infiltration
with time for contrasting soil textures.
Table 2. Soil characteristics for infiltration exercise in Introductory Soil Science (SC240).
Mapping unit
or soil name
Soil taxonomy
“Play” sand
Terry very fine
sandy loam
Bresser sandy loam
Nunn clay loam
unclassified
coarse-loamy, mixed, superactive,
mesic Ustic Haplargids
fine-loamy, mixed, superactive,
mesic Aridic Argiustolls
fine, smectitic, mesic Aridic Argiustolls
A
(Table 1)
C2 (from regression
analysis of Eq. [1]; see Fig. 6)
z, hydraulic conductivity
(cm sec−1; estimated from
Eq. [2])
2.37
0.17
5.5 × 10−2
10.88
0.038
3.4 × 10−3
5.36
0.012
2.3 × 10−3
9.11
0.0030
3.3 × 10−4
Table 3. Infiltration exercise completed by Introductory Soil Science students working in groups of three or four.
Infiltration and hydraulic conductivity experimental procedure
Objective is to measure cumulative infiltration and estimate hydraulic conductivity using a Decagon Disk Infiltrometer.
1. Each group of students will measure cumulative infiltration in a soil using a 2.0-cm tension head Decagon Mini-Disk Infiltrometer at 15-second intervals for 5 minutes (300 seconds).
2. Fill the disk Infiltrometer by complete submergence in tap water and inserting the stopper while the infiltrometer is under water. Then set the infiltrometer to the 0 mL mark by placing the porous plate on a paper towel.
3. Select individuals to complete the following tasks: (i) observe the water level in the infiltrometers for each 15-second interval, (ii) record the water
level for each time interval on Data Sheet 1, (iii) time each 15-second increment.
4. Secure the infiltrometers in the clamp attached to a ring stand (Fig. 2).
5. Lower the clamp plus infiltrometer to the surface of the soil (Fig. 3) and immediately begin timing.
6. Record the milliliters of water indicated in the infiltrometers for each 15-second increment on Data Sheet 1.
7. Complete the calculations for the table on Data Sheet 1.
8. Plot the centimeters of cumulative infiltration (y axis) vs. time in seconds (x axis) on the graph provided on Data Sheet 2.
9. Working with your lab instructor, complete a regression analysis of your cumulative infiltration data to fit the following statistical model.
Cumulative cm = C1(time)0.5 + C2(time)
where C1 and C2 are curve-fitting parameters.
10. Calculate the hydraulic conductivity using the following relationship:
K = C2/A
where K = hydraulic conductivity in cm/sec; C2 = curve-fitting parameter from regression analysis in Step 9; A = van Genuchten parameter (based
on soil texture). Your lab instructor will provide the A value.
Record your result on Data Sheet 1.
11. Answer the questions on Data Sheet 3.
J. Nat. Resour. Life Sci. Educ., Vol. 34 2005 • 73
Data Sheet 1
A. Infiltration data
Soil texture =
Time, seconds
(Time)0.5
0
0
15
3.87
30
5.48
45
6.71
60
7.75
75
8.66
90
9.49
105
10.2
120
11.0
135
11.6
150
12.2
165
12.8
180
13.4
195
14.0
210
14.5
225
15.0
240
15.5
255
16.0
270
16.4
285
16.9
300
17.3
B. Hydraulic conductivity = __________ cm/second.
Volume, mL
0
Name:
Lab section:
Cumulative infiltration, cm†
0
† cm = Volume (in mL or cm3)/(3.14 × 1.59 cm2).
Data Sheet 2
C. Cumulative Infiltration Graph
Data Sheet 3
Cumulative Infiltration Graph
D. Questions
1. Describe how your cumulative infiltration curve changed with increasing time and why these changes occurred.
2. Would you expect a higher hydraulic conductivity in a soil high in
clay or high in sand? Briefly explain your answer in terms of soilpore properties.
Cumulative Infiltration, cm
3. What force(s) are involved in soil–water movement when you first
add water to the dry soil?
4. What force(s) are involved in soil–water movement when the soil
approaches saturation?
5. List three factors other than soil texture that could increase the hydraulic conductivity of a soil.
6. Did the infiltration curves for your lab section follow the pattern you
expected based on soil texture? If not, explain why.
0
30
60
90
120
150
180
210
240
270
300
Time, seconds
Haplargids) (Table 2) after students completed the infiltration exercise. The Terry soil was the only soil in which we could remove the
wetted soil intact. This showed the three-dimensional wetting and
how the water had moved through the dry soil (see Fig. 4 and 5).
On the formal course evaluation for Introductory Soil Science
Laboratory (SC240L) at the end of the Fall 2004 semester, we asked
students in four out of eight lab sections (60 students) to provide
anonymous written comments on the soil infiltration exercise. We
plan to use this information to improve the exercise.
RESULTS AND DISCUSSION
After each student group finished the infiltration exercise, we
projected one graph containing all SigmaPlot infiltration curves and
associated regression analyses (Fig. 6) through a computer projec74 • J. Nat. Resour. Life Sci. Educ., Vol. 34, 2005
tion system onto a screen. We then discussed the resulting differences for the soils. For example, we prompted the students to discuss
what may have caused the Terry very fine sandy loam to have larger
cumulative infiltration than the Bresser sandy loam (fine-loamy,
mixed, superactive, mesic Aridic Argiustolls). We believed showing
the results for all soils on the same graph allowed students to understand some of the infiltration physics.
After extracting the wetted soil in the Terry very fine sandy loam
container, we had each student group observe the three-dimensional
pattern of soil wetting. We pointed out the water movement was
equidistant in every radial direction from the water source. We cued
them to think of what forces would pull the water equally in all
directions.
Even after showing the three-dimensional nature of wetting front
and emphasizing the importance of capillary forces, about 75% of
Fig. 2. Securing the infiltrometer to pinch clamp and positioning it above
the soil surface.
Fig. 4. Top view of the Terry very fine sandy loam wetting front after
wetted soil is removed from the plastic container.
Fig. 3. Contacting the porous plate to the soil surface and initiating infiltration
Fig. 5. Side view of the Terry very fine sandy loam wetting front after
wetted soil is removed from the plastic container.
Table 4. Selected direct quotes, with original punctuation and spelling, from Introductory Soil Science Laboratory (SC240L) students from the
fall of 2004 regarding the soil infiltration exercise.
1. I liked the infiltration lab, but it would have been nice to have been able to watch all the samples to be able to compare them.
2. The infiltration was good; it was good to actually see how different soils have different infiltration rates.
3. The infiltration was probably the best, because we learned the most from what we were doing.
4. I enjoyed the infiltration project. It was very interesting to see the differences between the different soils.
5. Infiltration lab was really good. It would have been nice to see the infiltration of water into other soils, but it was good to see the numbers.
6. I found that the infiltration exercise was fun and educational, and tied in to the lecture well. You should keep it in this course.
7. As for the new labs, infiltration was cool and it is good to know.
8. The infiltration lab was pretty fun. It was neat to dig out the infiltration front with the poop scoop and see its shape.
9. The infiltration lab was a very fun lab to do. It was my favorite lab that I did.
10. The new lab is very good; it is one of the most memorable.
11. Thought the lab with soil permeability was interesting and should be repeated.
12. The lab with perculations was very informative and fun.
13. The infiltration exercise was awesome. I did enjoy it.
14. The infiltration exercise I thought was a very good lab procedure and I feel that I learned a lot from it. You should use this lab procedure in the future.
15. I really liked the infiltration lab, it made a good point about how pore size affects H2O movement—it showed that what we learned in class really
does happen!!
16. The infiltration exercise seemed to be the most hands on and most involved in lab. Fun to put the information to good use.
J. Nat. Resour. Life Sci. Educ., Vol. 34 2005 • 75
provide written comments about the infiltration exercise. Out of an
enrollment of 60 in these four lab sections, 35 students responded
(58%). All student responses indicate that the exercise was well received; we did not receive any negative comments.
We believe the soil infiltration exercise successfully engaged
students in active, group learning that improved student understanding of soil–water flow dynamics. The equipment used is relatively
inexpensive and easy to operate. Student comments point out that
students enjoyed the exercise primarily because they were actively
involved and they were able to compare the data for different soils
(Table 4). Two students made the excellent suggestion to set up the
lab so that all students could see the infiltration into all soils at the
same time (Table 4; Comments 1 and 5). We recommend the use
of the Decagon Mini-Disk Infiltrometers as a tool to help introductory soil science students comprehend the soil physics involved with
infiltration.
Fig. 6. Cumulative infiltration curves and associated regression models
for four different soils.
REFERENCES
the SC240L students (122 students enrolled in fall of 2004) realized that capillary forces were most responsible for water movement
when water was initially added to the dry soil (see Question 3 in
Table 3). Similarly, about 75% of the students also recognized that
gravitational forces were most responsible for water movement as
the soil approached saturation (see Question 4 in Table 3). The majority of students were able to answer the remainder of the questions
in Table 3 correctly. We hope to improve this exercise to increase the
level of understanding for both of these concepts. Possible improvements include (i) use of a wider range of soil textures, (ii) attempt
to use soils with their natural structure, and (iii) have all infiltration
tests simultaneously in one location so that students could observe
all flow-rate differences at a glance.
We provide representative student comments for fall of 2004
in Table 4. In four of our eight lab sections, we asked students to
Carsel, R.F., and R.S. Parrish. 1988. Developing joint probability distributions of soil water retention characteristics. Water Resour. Res.
24:755–769.
Decagon Devices. 1998. Measuring soil hydraulic conductivity with a disk
infiltrometer. Available at www.decagon.com/manuals/Infiltrometer.
pdf (accessed 30 Nov. 2004; verified 11 July 2005). Decagon Devices,
Pullman, WA.
Helsel, D.G., and L.B. Hughes. 1984. Urban students in agriculture: The
challenge. J. Agron. Educ. 13:31–33.
Philip, J.R. 1957. Theory of infiltration: I. The infiltration equation and its
solution. Soil Sci. 85:345–358.
Thien, S.J. 1973. A learning center approach to introductory courses. J.
Agron. Educ. 2:75–79.
van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–
898.
Zhang, R. 1997. Determination of soil sorptivity and hydraulic conductivity
from the disk infiltrometer. Soil Sci. Soc. Am. J. 61:1024–1030.
76 • J. Nat. Resour. Life Sci. Educ., Vol. 34, 2005